1. Field of the Invention
This invention relates to a process for the production of a polyacetal copolymer. More particularly, this invention relates to a process for economically producing a polyacetal copolymer excelling in such qualities as thermal stability by a simple procedure of copolymerizing trioxane as a main monomer and a comonomer copolymerizable therewith in the presence of a specific nonvolatile protonic acid catalyst as a polymerization catalyst thereby enabling unaltered monomers occurring in the latter stage or the final stage of the polymerization to be efficiently and economically removed from the polymerization system and recovered for reuse.
2. Description of the Prior Art
As a process for the production of a polyacetal copolymer, the cationic polymerization using trioxane as a main monomer and a cyclic ether or cyclic formal possessing adjoining carbon atoms as a comonomer has been known. As cationically active catalysts for use in the polymerizations of this class, Lewis acids, particularly halogenides of boron, tin, titanium, phosphorus, and antimony such as, for example, boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorus penta-chloride, phosphorus pentafluoride, bismuth pentafluoride, and antimony pentafluoride, and such compounds as complex compounds or salts thereof, protonic acids such as, for example, perfluoro-alkyl sulfonic acids and perchloro acids, esters of the protonic acids, particularly esters of perchloro acids with lower aliphatic alcohols such as, for example, perchloro acid-tertiary butyl esters, protonic anhydrides, particularly mixed anhydride of perchloro acids with lower aliphatic carboxylic acids such as, for example, acetyl perchlorate, or trimethyl oxonium hexafluoro-phosphate, triphenylmethylhexafluoroarsenate, acetyl tetrafluoroborate, acetyl hexafluorophosphate, and acetyl hexafluoroarsenate have been proposed.
Among other catalytically active catalysts mentioned above, boron trifluoride or a coordination compound of boron trifluoride with an organic compound such as, for example, an ether is most widely utilized on a commercial scale as the catalyst for the polymerization having trioxane as a main monomer.
No matter which catalyst may be adopted, the rate of polymerization decreases so suddenly in the latter stage of polymerization as to render it extremely difficult to obtain quickly a yield of polymerization approximating closely to 100%. The polymerization consumes a very long time and, therefore, proves inefficient. In the latter stage of polymerization, the catalyst rather predominantly manifests an action of promoting the decomposition of the formed polymer and results in not only decreasing the molecular weight but also degrading such qualities as thermal stability. When the amount of the polymerization catalyst is increased, the overall rate of polymerization is indeed exalted. This increase, however, does not necessarily prove a proper measure for the overall process of polymerization because the catalyst in the increased amount further aggravates the degradation of the quality of the produced crude polymer and calls for a complicated stabilizing treatment in the latter stage.
The production of a polyacetal copolymer, therefore, has heretofore resorted generally to a method which comprises stopping the polymerization at a stage of relatively low conversion by adding to the polymerization system a solution containing an agent for inactivating the catalyst, washing the unaltered monomer remaining in the polymerization system, recovering the monomer in a refined form, and putting the recovered monomer to reuse. Since the unaltered monomer washed in this method is recovered in the form of a solution containing the monomer at a relatively low concentration, the reuse thereof necessitates use of a complicated process and consumption of energy for the sake of separation and purification. When the recovery of the unaltered monomer is abandoned, the unaltered monomer is completely wasted. In any event, the unaltered monomer turns out to be an economic burden.